1. Field of the Invention
The present invention relates generally to the treatment of subterranean formations with delayed crosslinking gel fluids, and more particularly, but not by way of limitation, to a treatment method wherein an aqueous gel fluid prepared on the surface crosslinks after being introduced into a wellbore which penetrates a subterranean formation to form a high viscosity treating fluid therein.
2. Description of the Prior Art
A variety of subterranean formation treatments utilizing crosslinked aqueous gels have been developed heretofore. For example, a well known technique for stimulating the production of oil and/or gas from a subterranean formation using a high viscosity crosslinked aqueous gel involves fracturing the formation. That is, the aqueous gel is injected down the wellbore penetrating the formation at a flow rate and pressure sufficient to develop hydraulic forces which fracture the formation. Continued pumping of the fluid containing propping agent, e.g., sand, extends the fractures and deposits the propping agent in the fractures to hold them open. Other treatments using high viscosity crosslinked aqueous gels include the temporary plugging or blocking of portions of a formation, enhancing the recovery of oil or gas from subterranean formations by the injection of high viscosity flooding fluids therein, etc.
In order to avoid the necessity of pumping crosslinked gel fluids from the surface to a subterranean formation to be treated, delayed crosslinking gel fluids have heretofore been developed. That is, instead of incurring the high surface pressures and horsepower requirements involved in pumping highly viscous crosslinked gel fluids, the crosslinking of the fluids is delayed until after the fluids have been pumped into the formation. Once in the formation, crosslinking occurs and useful high viscosity is developed.
In some applications, and particularly in carrying out subterranean formation fracturing treatments, it is highly advantageous to combine carbon dioxide with the aqueous gel fluid used. The purpose of the carbon dioxide is to reduce the water content of the treating fluid and to energize the fluid whereby upon the termination of the treatment, it readily flows back from the formation and is recovered therefrom. The particular quantity of carbon dioxide utilized with the aqueous gel fluid can range from commingled fluids containing from about 5% to about 50% by volume carbon dioxide to foamed fluids containing up to in excess of 96% by volume.
A fracturing method for stimulating subterranean formations utilizing foams comprised of aqueous gels and carbon dioxide is described in U.S. Pat. No. Re. 32,302. U.S. Pat. No. 4,488,975 is directed to a fracturing method which utilizes an aqueous gel containing a crosslinking agent and carbon dioxide. The crosslinking agent is a zirconium or aluminum chelate, and a sufficient quantity of carbon dioxide is used to reduce the pH of the aqueous gel to a level below about 5.5 whereupon crosslinking occurs. U.S. Pat. No. 4,657,081 is directed to the delayed crosslinking of a fracturing fluid comprised of an aqueous gel and carbon dioxide. A crosslinking agent is combined with a chelating agent resulting in a chelate solution having a specific pH. When combined with an aqueous gel fluid, the chelate solution causes a delayed crosslinking to take place.
By the present invention, an improved method of treating a subterranean formation with a delayed crosslinking gel is provided which can be utilized with or without carbon dioxide, and which results in the formation of crosslinked gels having good stability at high temperatures.